I have read the comments re various foods, & (possibly not wholly definitive), prognosis & diagnosis. I am very mindful the dog has been opened up with some invasive exploratory work, & testing going on.A big red flag for me is you say your vet has stated they do not think your dog has food allergies yet you have been advised to feed a well known vet pushed HA food !!!!
Don't rule out a K.I.S.S.
( Keep It simple stupid)
..........if it sits better replace the word stupid with "strategy".
To me, in back of my mind I was be thinking along lines of.............
Young dog + Scavenging + Fussy eating + Food changes &/or wrong amounts = Diarrhea
Colitis, Giardia , IBD (or maybe IBS)........the result is the same........... Diarrhea
...........How would I deal with it ?Dealing with dog Diarrhea
1. Supervise all activity to eliminate scavenging, ( this includes eating mud, garden matter faeces etc)
2. Starve the dog & give water only for 24hrs or maybe a bit longer
3. Feed one bland white meat protein & that alone (some say add rice - in my experience it can hinder recovery)
4. Monitor output, Cut back or revert to 2 if loose again....
5. If ok gradually re-introduce original food.
6 IIf still loose after 3 days consider Vet &/or AB's etc Managing a True Elimination diet
Only feed one bland protein source NOTHING else
White fish for 10 days
Cooked chicken 10 days
Cooked beef 10 days
Monitor ( output, skin, eyes, condition, itching ) etc etc etc
If you manage to find 3 tolerable protein sources you have more than enough to use these as a base for meals. Apply similar process if introducing other stuff to the bow.
You again speak of other food brands....the compositions of bought foods have many many ingredients. If you feed a certain brand with multiple ingredients in composition you may have no idea what the problematic one is in the event dog becomes loose on it...Dottie is the go to for home cooked advice.
Your thoughts have already swayed from air dried (ziwipeak air dried claims to be good but very expensive) & now mention cold pressed & the brand Gentle.........If later looking at cold pressed, wanting a decent food from a trusted brand with great ongoing customer support you will struggle to do better.....With cold pressed foods the binders get reckoned as carbs on here. ...Some cold pressed foods rely on potato flakes others use mineral clays etc etc. Beth @ Gentle may be able to explain more.
It may well be that you have a dog with a multitude of various issues going on BUT IMO you are massively overthinking things & adding far too many variables........wet, dry-extruded, air dried, home cooked , cold pressed, fillers, not to mention Cassia gum (I had never heard of before your post & Davids response)
If the dog is currently doing okay - maybe don't rush to do anything !